With the rapid development of mobile communication and the Internet, a large number of mobile multimedia services have emerged, and in some of the services, such as video on demand, television broadcast, video conference, online education, and an interactive game, it is necessary for a plurality of users to receive the same data simultaneously. These mobile multimedia services are characterized by a large amount of data and long duration, compared with the common data services. In order to utilize mobile network resources efficiently, 3GPP, which is a global organization for standardization, has proposed Multimedia Broadcast/Multicast Service (MBMS). MBMS refers to a point-to-multipoint service for sending data to multiple users by one data source, which implements sharing of network resources including the mobile core network and access network, especially sharing of the air interface resource. Unlike Cell Broadcast Service (CBS) in the existing mobile network, which allows data of a low bit rate to be sent to all users through a cell shared broadcast channel and is a service based on message, MBMS defined by 3GPP can implement not only the message type multicast and broadcast of plain text at a low rate, but also the multicast and broadcast of a multimedia service at a high rate, such as a mobile TV service.
However, in the system defined by the original 3GPP Release 6, MBMS has a low spectrum efficiency, which is typically 0.2 to 0.02 bit/Hz/s. Therefore, in Long Term Evolution (LTE) project, 3GPP starts the study on Enhanced Multimedia Broadcast/Multicast Service (E-MBMS), and has introduced the Single Frequency Network (SFN) technology from the existing industrial terrestrial broadcast standard (e.g. DVB-H, MediaFLO, etc). Particularly, in all cells or sectors in the SFN, the same MBMS is sent utilizing the same physical resources (including time, frequency, a code channel, a scrambling code and midamble) at the same time, thus a User Equipment even at the edge of the cell or sector can receive useful signals from different cells or sectors, and may merge energy of all the received useful signals at an air interface directly. Therefore, Quality of Service (QoS) of MBMS in the whole SFN area can be improved.
Referring to FIG. 1, which shows a network structure topology of an SFN. Now the existing implementation of the SFN technology in E-MBMS of 3GPP will be described by way of example, based on the network structure of the SFN (where each cell is composed of three sectors, and the base stations of the three sectors share the same address) shown in FIG. 1.
The same time resource, frequency resource, scrambling code and midamble used by a broadcast/multicast service are allocated to all the sectors in the SFN shown in FIG. 1, and all User Equipments (UEs) in the SFN also employ the allocated time resource, frequency resource, scrambling code and midamble to receive the broadcast/multicast service. That is, from the perspective of a UE, as long as signals from the sectors in the SFN fall into the window of a multi-path receiver of the UE, the UE can directly merge energy of all the signals falling into the receiving window, thereby greatly enhancing the performance of receiving the broadcast/multicast service.
Now an implementation of transmitting a multimedia broadcast/multicast service in the SFN in the prior art will be described by way of example.
To transmit a broadcast/multicast service in a plurality of cells or sectors, a Radio Network Controller (RNC) allocates the broadcast/multicast service resources to the cells or sectors, assigns the scrambling code and midamble (which are different from the existing scrambling code and midamble) used for the broadcast/multicast service, and notifies each Node B and UE about the allocated broadcast/multicast service resources and the assigned scrambling code and midamble via signaling. The Node B of each cell or sector employs such particular codes to form a broadcast/multicast service burst and transmits the burst over the same time and frequency resources. The UE receives at the corresponding resource location the same bursts transmitted simultaneously by a plurality of cells or sectors, then performs channel estimation using the assigned midamble, and descrambles the data using the assigned scrambling code to obtain the desired broadcast/multicast service data.
The implementation of transmitting the multimedia broadcast/multicast service in the SFN in the prior art includes the following main processes.
Process 1: The network side initiates a broadcast/multicast service and notifies an RNC by signaling. The RNC determines cells or sectors in which the broadcast/multicast service is to be transmitted.
Process 2: The RNC allocates broadcast/multicast service resources to the determined cells or sectors. In order to facilitate the implementation of macro diversity, the RNC may allocate the same time and frequency resources (e.g. the same frequency point, slot and code channel) and assign the scrambling code and midamble used for the broadcast/multicast service.
The assigned scrambling code and midamble herein are different from those used for the non-broadcast/multicast service.
A group of scrambling codes and midambles in close correlation with the existing scrambling code and midamble may be preset for the broadcast/multicast service and used as the dedicated scrambling codes and midambles for the broadcast/multicast service, thus forming a broadcast/multicast service code group table, which may be stored in the RNC, UE and Node B. Thus, when allocating resources for a certain broadcast/multicast service, the RNC may select a pair of codes from the code group table and notify the Node B and the UE of the corresponding code group identifier, thus reducing the signaling load.
Process 3: When transmitting the broadcast/multicast service, the Node B forms a broadcast service burst using the assigned scrambling code, midamble and corresponding data, and transmits the broadcast service burst over the resources allocated by the RNC.
Process 4: According to the resource allocation information sent by the RNC, the UE receives the signals over the corresponding resources. The UE may receive the signals sent by a plurality of cells or sectors, directly superimpose all the received useful signals at the air interface, and perform channel estimation according to the assigned midamble to obtain the total channel estimation result from the plurality of cells or sectors to the UE, then de-spread the data according to the channel estimation result, descramble the de-spread data using the assigned scrambling code, thereby obtaining the desired broadcast/multicast service data.
It can be seen from the above processes that the RNC only needs to assign the scrambling code and midamble used for the broadcast/multicast service burst when allocating resources and notify the same to the Node Bs and the UE, and the Node Bs and the UE respectively send and receive the broadcast/multicast service data according to the assigned scrambling code and midamble, in this way, the transmission of the multimedia broadcast/multicast service can be implemented easily, thus the quality of the broadcast/multicast service signal received by the UE is improved, and the coverage area of the broadcast/multicast service is enlarged. In addition, the broadcast/multicast service is typically transmitted in hot cells or sectors, which generally have small radiuses, therefore the time difference between the signals received by the UE from different cells or sectors can not be too large, which has no high demand for the physical layer process of the UE.
The above technology greatly improves the transmission of traditional MBMS, but has some defects still to be solved. The deep fading phenomenon caused by the superimposition of signals received from a plurality of cells or sectors is not solved yet. In the deep fading phenomenon, since the same signals are transmitted, for example, in two cells or sectors, if the signals arrive at the receiver of the UE simultaneously but have opposite phases, the quality of the received signals will be deteriorated very seriously. This phenomenon is particularly significant in the stationary environment, in the low speed environment and at the edge of two adjacent sectors at the same station address.